In the past I've made a lot more $$$ with them than without using them.
That said there is a lot more to SEO today then most realise,the seo plugins help to remind you the basics of on page SEO as you write posts.
If you still have a new domain with no relevant backlinks,high bounce rate,slow site load speed and much more the best seo plugin will do next to nothing for you.
There is no magic bullet,you still need knowledge and experience no matter which plugin you use.
Have a read of this